Elastic polypropylene film and process



Sept. 22, 1970 s. CQOPPENLANDER 3,530,103

ELASTIC POLYPROPYLENE FILM AND PROCESS Original Filed June 12, 1964GEORGE C. OPP ANDER ENTOR.

' ATTORNEY United States Patent 3,530,108 ELASTIC POLYPRUPYLENE FILM ANDPROCESS George C. Oppenlander, Embreeville, Pa., assignor to HerculesIncorporated, Wilmington, Del., a corporation of Delaware Originalapplication June 12, 1964, Ser. No. 374,634. Divided and thisapplication Jan. 26, 1968, Ser.

Int. Cl; C0815 29/02 US. Cl. 260--93.7 1 Claim ABSTRACT OF THEDISCLOSURE An elastic polypropylene film is taught characterized bygamma orientation and exhibiting an elastic recovery of about 80 to 91%from a 75% elongation. The polypropylene is modified by incorporation ofabout 0.1 to 0.8 mol of an azido crosslinking agent per mole ofpolypropylene.

This application is a division of copending US. application Ser. No.374,634, filed June 12, 1964, now Pat. No. 3,382,306.

The present invention relates to an elastic film of stereoregularpolypropylene and to a process for the production of such film.

It is known to produce stereoregular polypropylene yarn which has acertain degree of elasticity, such yarns and the processes for makingthem being disclosed, for example, in British Pat. 935,809. According tothat patent, the yarns are characterized as having gamma orientation, aheat stable orientation angle of to 30 and a tensile recovery on thesecond and succeeding cycles from a 25% elongation of at least 85%. Theyarns preferably also have a gamma intensity ratio of at least 1. Thepatent, however, is concerned solely with the preparation of elasticyarn and gives no indication as to how one might make elastic film fromthe same polymer. This is not surprising inasmuch as the manufacture ofa film involves distinctly different considerations from those involvedin the manufacture of a fiber.

Elastic polypropylene films find many applications in the packagingfield. Bands of such a film are useful, for instance, as an inexpensivesubstitute for rubber straps in many applications. An elastic film isuseful as a covering for a window in a package because its resiliencemakes it more resistant to breakage when it is handled. Such films arealso used as form fitting wrappers for a variety of articles.

In accordance with the present invention, there is provided a novel filmof stereoregular polypropylene, also known as crystalline or isotacticpolypropylene, which film is characterized by gamma orientation and atensile recovery from 75 elongation of at least 80%. The invention alsoprovides a novel process for making this film which process comprisesextruding molten stereoregular polypropylene at a temperature between220 and 290 C. in the form of a film, subjecting the extruded film to amelt draw of at least 24 to 1, quenching the film in an inert liquid ata temperature of from 80 to 110 C. within A2. to inch from the die, andheat treating the solidified film in the relaxed state at a temperaturebetween about 130 and 155 C. According to an optional embodiment of theprocess, there can be incorporated into the polymer prior to extrusionan azido cross-linking agent, the presence of which leads to still'further improvement in the elastic properties of the film. The azidocross-linking agent in the latter embodiment is used in a quantitysufficient to increase the melt viscosity of the polymer butinsufiicient to affect its solubility p in perchloroethylene at 110 C.This quantity is usually Percent R= x where L represents the distancethe sample was elongated, and L represents the portion of thatelongation which is retained upon release of the elongating stress. Theparameter L may also be referred to as permanent set. The percentage ofpermanent set is equal to 100 minus percent recovery. It should beobvious that higher values of recovery, or conversely, lower values ofpermanent set indicate greater elastic properties.

The process of the invention is illustrated in the attached drawing inwhich 1 designates an extruder adapted to extrude molten polymer 2through a die 3. The numeral 4 represents a vessel containing aquenching liquid 5, e.g., water, in which is submerged an idler roll 6.Numerals 7 and 8 designate nip rolls which convey the film through thequenching liquid, 9 designates a heat treatment oven, containing aseries of idler rolls 10. 11 and 12 designate an additional set. of niprolls and 13 designates a Wind up station.

In operating according to the drawing, the molten polymer 2 is extrudedin the form of a film 14 into the inert liquid 5 in the quench tank 4.The film is drawn through the quench tank by the nip rolls 7 and 8 whichare positively driven at a rate 24 to 70 times the linear rate ofextrusion of the film through the die. This increase in the linear speedof the film causes stretching of the film in its weakest section, i.e.,the molten portion immediately following the die. The inert liquid inthe quench tank through which the film is drawn is maintained at atemperature sufficient to solidify the film but which causes it to coolrelatively slowly. A satisfactory temperature is from about 80 to C. Thequenched film is drawn from the tank and fed into the heat treatmentoven 9 and around idler rolls 10. In passing through this oven, the filmis maintained free or substantially free of tension at a temperaturebelow its softening point. A second set of nip rolls 11 and 12 at theexit of the oven are driven at the same speed as the inlet nip rolls inorder to move the film through the oven. with a minimum of tensionthereon. The finished film is then wound into rolls at wind up station13.

The extruded film is subjected to a melt draw of at least 2400% or,stated otherwise, a melt draw down of at least 24 to 1. By melt drawdown is meant the reduction in thickness of the extruded film while itis still in the liquid, or molten, state prior to quenching. Melt drawratio expresses the relationship between the die opening and theultimate thickness of the film. Melt draw down is commonly employed inthe production of films and fibers from thermoplastic polymer forpractical reasons. For example, the inclusion of the melt drawn clownpermits the use of a larger size die than can be used otherwise. Thispermits greater precision in die manufacture and results incorrespondingly greater gauge uniformly in the extruded films. v

In the process of this invention, the use of a high degree of melt drawndown is found to be vital to the formation of elastic film. In order toproduce the elastic films of the instant invention from unmodifiedstereoregular polypropylene, the film must be subjected to a draw downof at least about 24 to 1 in the molten state as it leaves the die. Theupper limit of melt draw down is imposed only by film breakage.Ordinarily, the problem of film breakage is found to become acute atabout 70 to 1 draw down. When the polypropylene has been modified withan azido cross-linking agent as previously noted, the amount of meltdraw down required is substantially less, though substantial melt drawdown is still required. This will be more fully discussed hereinafter.

When melt drawing the film by this relatively large amount, it isnecessary to confine the drawing to a relatively small distance. If thedraw down takes place over any sizeable distance, the resultant filmexhibits an unacceptable amount of gauge variation. To assure that thedrawing is thus confined, it is necessary to quench the film to thesolid state within a relatively short distance of the die. Since thefilm will draw preferentially in the molten areas, rapid quenching tendsto isolate the draw area. To obtain the optimum in gauge uniformity, thefilm must be quenched within 4 inch of the die.

The quenching of the film is effected by immersing the film in a bath ofcooling liquid, e.g. water, maintained at a temperature of about 80 to110 C. It is desirable that the film not be permitted to cool toorapidly,

thus the relatively high quench bath temperature. The I reason whybetter elastic properties in the film result when the polymer isquenched slowly is not known for certain, but it is believed that thisis related to the type of crystal structure which forms with slowcooling, it being known that dilferent crystal structures can be formedin stereoregular polypropylene depending upon the rate at which thepolymer is cooled from the melt.

The film which is formed by the extrusion, melt draw down, and quenchingoperations discussed above has very little elasticity as it is procuredfrom the quench bath. The latent, inherent elasticity is fully developedby a tension free heat treatment at a temperature in the range of about130 to 150 C. for a minimum time of about 3 minutes. The treatment isintended to relax any strains which may have been set up within the filmas a result of the melt draw down. Thus, it is essential that the filmbe in a tension free state when it is so treated in order to allowrelaxation. This may be accomplished very effectively on a continuousbasis, as, for example, by the use of overdriven rolls in drawing acontinuous advancing sheet of the film through a treatment chamber.

After heat treating the film of unmodified polypropylene to develop thelatent elasticity, the film is capable of at least 85% recovery afterelongation of up to about 75% in the machine direction.

Further improvements in the elasticity of the films of the instantinvention result from the presence in the starting polymer of an azidocross-linking agent. When the polymer is modified by an azidocross-linking agent, it is found that the required amount of gammaorientation is imparted to the polymer by a much lower melt draw down.Melt draw down of about to to 1 has been found to yield a satisfactorydegree of gamma orientation to produce elastic film when an azidocompound is present. The lower melt draw is advantageous since higherdraw down is frequently accompanied by an increased number of filmbreaks.

Additionally, the azido cross-linking agent-modified polymers producefilms which exhibit an even higher degree of elasticity when subjectedto the other process steps of the invention than do the regularunmodified polymers.

The azido cross-linking agents are exemplified by the sulfonazides andthe azidoformates. The sulfonazides have the general formula:

Where R is an aliphatic or aromatic hydrocarbon radical inert toreaction with polypropylene and x is an integer greater than 1.Exemplary sulfonazides are 1,5-pentane bis(sulfonazide), 1,10-decanebis(sulfonazide), 1,3-benzene bis(sulfonazide), 1-octyl-2,4,6-benzenetris(sulfonazide), 4,4-diphenylmethane bis(sulfonazide), 4,4-diphenylether bis(sulfonazide), 4,4-bis-octadecylbiphenyl-3,-5,3,5-tetra(sulfonazide), 4,4'-diphenyl disulfide bis(sulfonazide), 1,6-bis(4'-sulfonazidophenyl) hexane, 2, 7-naphthalenebis(sulfonazide), etc. Other sulfonazides may be used, for instancethose having functional groups on the organic hydrocarbon such ashalogen, carboxyl groups, carbonyl groups, etc., so long as such groupsare inert to reaction with polypropylene. The azidoformates have thegeneral structural formula:

where R is an organic radical inert to bridging reactions and x is aninteger from 2 to about *8. Exemplary of the azidoformates used are thealkyl azidoformates such as t etramethylene-bis(azidoformate),pentamethylene-bis(azido-formate); the cyclic alkyl azidoformates suchas 1,4 cyclohexanedimethyl bis(azidoformate); the aralkyl azidoformatessuch as a,a'-p-xylylene-bis(azidoformate); the aromatic azidoformatessuch as 2,2-isopropylidene-bis(p,pphenyl azidoformate); the azidoformateethers such as 2,2'-oxydiethyl-bis(azidoformate), 2,2'-oxydipropyl-bis(azidoformate), 2,2 ethylenedioxydiethyl-bis-(a-zidoformate) thetetraazidoformate of pentaerythritol-p ropylene oxide adduct having thegeneral formula:

CH (I? the azidoformate thioethers such as 2,2-thiodiethyl-bis(azidoformate), 4,4-thiodibutyl-bis(azidoformate); etc. It will, ofcourse, be obvious to those skilled in the art that still otherazidoformates containing functional groups, which are inert tomodification reactions, such as halogen, carboxyl groups, carbonylgroups, etc., are included in the above definition.

It is known to cross-link polypropylene by the use of azido compoundssuch as the aforesaid sulfonazides and the azidoformates. Suchcross-linked or vulcanized compositions exhibit a much higher impactstrength, durability, and stiffness than does normal polypropylene andare insoluble in perchloroethylene at C. The values attained by theseparameters are dependent upon the amount of the azido compound which isused to effect the cross-linking.

In order to improve elastic properties of the film of this invention, itis necessary that a small amount, and only a small amount of suchcross-linking agent be present. That is, the amount should be sufficientto increase the melt viscosity of the polymer but insutficient to alteris solubility in perchloroethylene at C. Generally, this amount ofcross-linking agent is provided by the addition of 0.1 to 0.8 mole ofthe azido compound per mole of polypropylene. When the amount of azidocompound materially exceeds this range, the polymer is too highlycrosslinked to exhibit the desired elastic properties. When the amountis below the range, little or no benefit is realized.

The azido cross-linking agent reacts with the polypropylene under theinfluence of heat. The temperature required to effect this reactiondepends somewhat .upon the particular azido compound being used, butwill usually be between about 100 and 250 C. This is easily effected bycontrol of the extruder temperature. That is to say, the polymercontaining the azido compound is extruded at a temperature higher thanthe temperature required to cause the reaction to take place so that thereaction occurs during extrusion.

The azido cross-linking agent can be added to the polypropylene by anymethod commonly used to accomplish the addition of an additive to apolymer. These methods include, but are not limited to, deposition onthe polymer flake from a solvent, mixing on a heated mill such as aBanbury mixer, and dry blending, or by deposition from a solvent.

In the examples which follow, film was produced essentially as outlinedabove using conditions outlined for each specific example. Recovery wasdetermined by extending the sample to the specified elongation, holdingit in the extended state for ten minutes, releasing it, and allowing itto relax for five minutes. Recovery is calculated as a prcentage of theimposed elongation which is not retained after relaxation for fiveminutes.

EXAMPLE 1 Particles of, stereoregular polypropylene having reducedspecific viscosity of 2.4 (determined in a 0.1% solution indecahydronaphthalene at 135 C.) which corresponds to a molecular weightof about 350,000 and stabilized against light and heat were wetted witha solution in acetone of a disulfonazide prepared from a chlorinated andsulfochlorinated mixture of petroleum hydrocarbons having an average of11 to 12 carbon atoms per mole and containing at least 0.1% chlorine(KSA). The solution was sufficiently concentrated to deposit 0.1% of thedisulfonazide on the surface of the polymer. The solvent was removed bygentle heating and reduced pressure. The polypropylene flake was meltedin a compounding extruder, and extruded at 233 C. at a rate of 1.33 feetper minute through a film die having a 40 mil die gap. The film wasdrawn away from the die at a rate of 32 feet per minute, resulting in adraw down of 24 to 1 and a film thickness of 1.7 mils. The film wasquenched in a water bath at 80 C. located A inch from the extrusion die.

A second batch of the stereoregular polypropylene flake was wetted withan acetone solution of tetramethylene bis(azidoformate) (TBAF), treatedas above, and extruded into a film under the same conditions asoutlined.

In addition, polypropylene containing no additive was extruded andsubjected to draw down of 24 to 1 and 67 to 1.

Samples of the resulting films were suspended, tension free, in a forceddraft oven at 150 C. for minutes. The films at this point had a highdegree of gamma orientation. The percent recovery of all these films isrecorded in Table 1.

The data in the table clearly show the elasticity developed in the filmsas a result of the process. The data also clearly show the improvedelasticity of the film made from the polymer containing the azidocross-linking agent. What -I claim and desire to protect by LettersPatent is:

1. An elastic film of isotactic polypropylene capable of about 80 to 91%recovery from an elongation of in the machine direction, said isotacticpolypropylene being characterized by exhibiting gamma orientation andcrosslinked with about 0.1 to 0.8 mole of an azido crosslinking agentper mole of polypropylene, said azido crosslinking agent being selectedfrom the group consisting of (a) R(SO N where R is an aliphatic oraromatic hydrocarbon radical inert to reaction with polypropylene and xis an integer greater than 1; and

where R is an organic radical inert to bridging reactions and x is aninteger from 2 to about 8.

References Cited UNITED STATES PATENTS 3,112,300 11/1963 Notta et al.26093.7 3,146,284 8/1964 Markwood 26421O 3,175,999 3/1965 Notta et al.26093.7 3,223,764 12/1965 Kahn et a1 264178 3,330,897 7/1967 Tessier264-176 3,256,258 6/1966 Herrman 260-93] 3,258,455 6/1966 Notta et al.260-937 JOSEPH L. SCHOFER, Primary Examiner E. 1. SMITH, AssistantExaminer US. Cl. X.R. 264178, 210

